The present invention relates to a soliton optical pulse generating device for high-speed fiber optic communication, an optical clock generating device for an optical information processing system, an ultra-high speed pulsed light source for photomerry and light source for optical sampling.
In the field of optical communication, an optical soliton communication has come to particular attention, with an increase in the transmission rate, as a substitute for conventional systems which use non-return-to-zero pulses that are susceptible to and adverse effect of wavelength dispersion and the influence of a nonlinear optical effect. The soliton optical communication is a system that positively utilizes the wavelength dispersion characteristic and nonlinear optical effect of optical fibers which are contributing factors to the degradation of characteristics of conventional transmission systems, and that transmits optical short pulses intact by balancing optical pulse spreading owing to the wavelength dispersion by the fiber and pulse compression based on the nonlinear optical effect. The optical short pulse is relatively easy of time multiplexing and wavelength multiplexing as well, and hence is suitable for high-speed transmission. This optical short pulse needs to be a transform limited pulse (a pulsed optical pulse of a spectrum corresponding to a Fourier transformation of an envelope waveform of an optical pulse); from the viewpoint of increasing the transmission rate, it is desirable that the limit repetition frequency of the optical pulse be as high as possible. To prevent interference between optical pulses, the pulses width may preferably be small. It is also necessary that the pulses be in susceptible to environmental conditions such as atmospheric temperature and like. Besides, the light source for optical sampling is required to be capable of changing the repetition frequency and the pulse width.
One possible means that meets with such requirements is a semiconductor electroabsorption optical modulator; since its extinction ratio, expressed in dB, bears a substantially linear relationship to the applied voltage, the semiconductor electroabsorption optical modulator is able to generate optical pulses having a virtually sech.sup.2 geometry, by launching laser light of a certain intensity into the modulator and applying thereto a sinusoidal modulating voltage. Moreover the phase modulation component of the semiconductor electroabsorption optical modulator is so small that the pulses therefrom are transform limited pulses (Literature 1: Masatoshi Suzuki, "Ultra-Short Pulses for Optical Soliton Transmission", Research on Laser, Vol 20, No. 8, pp. 673-683, 1992). This system is hard to be influenced by atmospheric temperature and similar environmental conditions and allows ease in changing the pulse repetition frequency and the pulse width. A system in which two modulation elements are connected in cascade under applying thereto rectangular waveforms was proposed in Japanese patent laying-open No. 143721/86. The highest repetition frequency of the conventional optical pulse generation is the operation limit frequency of the semiconductor electroabsorption optical modulator.